Fluids are heated in a variety of heating devices by heat generated through the combustion of a fuel. Such heating devices may have as their sole object the heating of the fluid itself such as in a boiler used to heat water to steam or as a reactor in which the heating of the fluid is necessary to support a reaction. A common reactor where such fluid heating is necessary is a steam methane reformer in which the fluid to be heated is a mixture of steam and hydrocarbons that is heated within passages filled with catalyst, to react and thereby form a synthesis gas.
Very often gas turbines exist in installations that have boilers, steam methane reformers, and/or other fluid heating devices. As may be appreciated, a gas turbine exhaust is a high temperature stream that can be advantageously used to augment the heating requirements of the fluid heating device to thereby lower fuel usage. However, in retrofit scenarios that involve replacing existing combustion air for steam methane reformers or fired heaters with gas turbine exhaust, several challenges are presented due to differences in oxygen content, temperature, pressure and flow rate in the gas turbine exhaust as compared with those of air. As will be discussed, addressing such challenges add to the expense involved in the integration which may make the integration unattractive in the first instance.
For instance, U.S. Pat. No. 4,784,069 discloses an air preheater to heat combustion air against gas turbine exhaust and thereby supply the heated air for combusting fuels in a radiant section of a steam methane reformer. This apparatus further includes a bypass around the air preheater to supply excess gas turbine exhaust to the convective section of the reformer for combusting fuel and/or for recovering heat. The degree to which fuel savings may be realized depends on the efficiency of the air preheater and its ability to transfer heat from the gas turbine exhaust to the incoming air. The fabrication costs of preheaters, however, increase with their efficiency. Therefore, the more fuel to be saved, the higher the acquisition cost for the air preheater.
In Vol. 113 Journal of Engineering for Gas Turbines and Power, “Cogenerative, Direct Exhaust Integration of Gas Turbines in Ethylene Production” by Cooke et al., pp. 212–220 (1991) direct integration options are disclosed in which ethylene cracking furnace combustion air is either completely or partially replaced with gas turbine exhaust. As pointed out in this paper, there has to be some means to divert exhaust gas during periods of lower furnace demand for oxygen and colder seasons in which the air is denser and hence, more oxygen is present. If this were not done, then the gas turbine would have to be operated at continuously varying loads, significantly less then full loads. This would lead to an unacceptable variable electrical output of a generator powered by the gas turbine. On the other hand, it would be a waste of fuel to simply divert part of the gas turbine exhaust and not recover its heating value. In order to solve this problem, a flywheel boiler is used that is capable of producing medium to low pressure steam from the excess exhaust. The use of such an economizer adds to the costs of the integration.
Stone & Webster Engineering Corporation, “Gas Turbine Integration to Reduce Costs,” by Shallice (1985) describes options for integrating gas turbine into a petrochemical complex. Either combustion air is fully replaced with gas turbine exhaust, or partially replaced in which case gas turbine exhaust is mixed with ambient air. The same problems regarding the use of excess turbine exhaust are present in this reference.
In U.S. Pat. No. 6,200,128, a method is disclosed in which oxygen and gas turbine exhaust is fed into a boiler. The use of oxygen, however, adds to the expenses involved in such an integration.
As will be discussed, the present invention provides a method and a system in which a gas turbine can be practically integrated with a fluid heater such as a steam methane reformer or a fired furnace without any modification of the fluid heater or oxygen addition, but with reduced fuel consumption.